1. Field of the Invention
The present invention relates to a device and a method for lighting a discharge lamp. In particular, the present invention relates to a device and a method for lighting a discharge lamp such that the life of the discharge lamp is prolonged.
2. Description of the Related Art
FIG. 10 is a circuit diagram showing a conventional discharge lamp lighting device. In FIG. 10, 1001 denotes a metal halide lamp used as a discharge lamp, and 1002 denotes a lighting circuit for starting/lighting the metal halide lamp 1001. The lighting circuit 1002 is composed of a d.c. power supply 1003, an inverter 1004, and a high-voltage pulse generator 1005. The d.c. power supply 1003 is composed of a rectifying/smoothing circuit 1007 and a step-down type chopper circuit 1029. The rectifying/smoothing circuit 1007 rectifies and smoothes the output of a commercial a.c. power supply 1006 so as to convert it into d.c. power. The step-down type chopper circuit 1029 includes a transistor 1008, a diode 1009, a choke coil 1010, a capacitor 1011, resistors 1012, 1013 and 1014, and a controller 1015. The transistor 1008 receives the output of the rectifying/smoothing circuit 1007 and controls the power which is supplied to the metal halide lamp 1001 at a predetermined value. The step-down type chopper circuit 1029 detects an output voltage by means of the resistors 1012 and 1013 and detects an output current by means of the resistor 1014, and performs a mathematical operation for the two detected signals at the controller 1015. Thus, the step-down type chopper circuit 1029 controls i.e., turns on or off, the transistor 1008 (based on the output signal from the controller 1015) so as to maintain the output voltage of the step-down type chopper circuit 1029 at a predetermined value. The invertor 1004 includes transistors 1016, 1017, 1018, and 1019 and a driver 1020. The output signal from the driver 1020 functions to alternately generate a period during which the transistors 1017 and 1018 are turned ON and a period during which the transistors 1016 and 1019 are turned ON. Thus, the output of the d.c. power supply 1003 is converted into a.c. power before being output from the invertor 1004. The high-voltage pulse generator 1005 generates high-voltage pulses for starting the metal halide lamp 1001.
Hereinafter, the operation of the discharge lamp lighting device of the above-mentioned configuration will be described. As the metal halide lamp 1001 is started by the high-voltage pulses generated by the high-voltage pulse generator 1005, a discharge arc forms between electrodes of the metal halide lamp 1001. After the metal halide lamp 1001 is started, a signal which is in proportion with the lamp voltage of the metal halide lamp 1001 is detected by the resistors 1012 and 1013, and a signal which is in proportion with the lamp current of the metal halide lamp 1001 is detected by the resistor 1014. These detected signals are subjected to a power control operation by the controller 1015, and the transistor 1008 is controlled, i.e., turned on or off, in such a manner that the power supplied to the metal halide lamp 1001 is maintained at a predetermined power level. The output of the d.c. power supply 1003 is converted into a.c. power by the invertor 1004 before being supplied to the metal halide lamp 1001. Thus, the metal halide lamp 1001 stays lit. The frequency of the a.c. current, converted from the output of the d.c. power supply 1003, is often set at a frequency which can avoid problems such as fluctuation or extinguishment of the discharge arc or bursting of the metal halide lamp 1001 due to an acoustic resonance phenomenon inherent to HID lamps.
However, the above-mentioned conventional technique is known to have the following problems. It is assumed that the metal halide lamp 1001 has electrodes A and B and that the high-potential-side output potential of the d.c. power supply 1003 is Va and the low-potential-side output potential of the d.c. power supply 1003 is Vb. FIG. 11 is a graph showing potential of electrodes used in the conventional discharge lamp lighting device. The electrodes A and B are each at a positive potential whose value shifts in a rectangular waveform. When the potential of the electrode A is Va, the potential of the electrode B is Vb; when the potential of the electrode A is Vb, the potential of the electrode B is Va. Thus, the average potential of the electrodes A and B (i.e., the average potential of the discharge arc) becomes (Va+Vb)/2. Since the minus-side potential of the lighting circuit is generally grounded, Vb is substantially zero. As a result, the average potential of the discharge arc of the metal halide lamp 1001 becomes positive with respect to the ground potential.
FIG. 12 is a diagram showing electric field in the conventional metal halide lamp 1001. Since it is likely that elements surrounding the metal halide lamp 1001 are maintained at the ground potential (that is, the average potential of the discharge arc becomes higher than the potentials of the surrounding elements), an electric field is generated in the direction of the elements, i.e., in the direction of the tube 103 wall of the arc tube from the discharge arc 106, i.e., from the discharge arc 106 toward outside, as indicated by the arrows in (a) and (b) of FIG. 12. A cross-sectional view taken on line II--II of (a) in FIG. 12 is shown in (b) of FIG. 12.
When the metal halide lamp 1001 is generating light, the light-emitting metals (e.g., Na and Sc) sealed within the arc tube are ionized so as to become positive ions having positive electric charge, and therefore are forced to move toward the tube wall due to the electric field generated in the direction of the tube wall from the discharge arc inside the discharge arc. Thus, the metal ions are likely to be moved toward the tube wall owing to the effect of the electric field generated inside the arc tube. As a result, the metal ion density increases in the vicinity of the tube wall.
On the other hand, the arc tube of the metal halide lamp 1001 is generally composed of quartz glass, which is known to have devitrification through reaction with metal ions. That is, an increase in the metal ion density in the vicinity of the tube wall increases the chances of the quartz glass reacting with the metal ions, thereby resulting in devitrification.